Remove cancan dependency for Spree 2.3.x
Remove cancan dependency for Spree 2.3.x

spree_core 2.3.0 and 2.3.1 already depend on cancan, while 2.3.2+
depends on cancancan, so the dependency should be resolved through
spree_core instead.

fixes #249
fixes #250

yomishra/auth_devise